FIG. 1 shows a typical three-phase high voltage cable arrangement CA for transmitting power from a first location A to a second location B including a first, second and third high voltage cable L1, L2, L3 for transmitting the respective currents of each phase R, S, T. A typical distance is 1-2 km.
The individual cables L1, L2, L3 have a typical construction of a high voltage cable as shown in FIG. 2. A conductor 1 is surrounded by an insulation 3 and the insulation is surrounded by a screen 5 and an outer coating 6. The screen can have a cross section of 35-50 mm2 and has a very high conductivity of ≈0,017 Ωmm2. Preferably, the screen is made of copper.
Furthermore, the conductor 1 may be surrounded by an inner conducting layer 2 and the insulation 3 may be surrounded by an outer conducting layer 4. These layers have low conductivity and are sometimes also called “semiconducting” layers. Such a cable construction is typically used for high voltages of >10 kV.
In a cable arrangement including three different cables L1, L2, L3 each having a construction as shown in FIG. 2 losses occur. The total losses can be divided into two types, namely losses that only occur if current is flowing in the cable system (current-related losses) and losses which are produced solely by the effect of the electrical field in the insulation (voltage-dependent losses). The current-dependent losses do not only occur in the current-carrying conductor itself (conductor losses) but also as so-called additional losses in the other metallic elements of the cable system where eddy and circulating currents are induced under the effect of the magnetic field of the current-carrying conductor. The conductor losses are for example Joule losses and also due to skin and proximity effects.
The most important additional losses are those caused by the axial induction currents in the metallic screen. Such additional losses in the cable sheath, screen and other metallic system components can be reduced by use of non-magnetic steel for armouring to prevent magnetic reversal losses, by grounding of the screens or metal sheaths at one end to avoid providing a closed loop for the induction currents, or by a so-called “cross-bonding” of the cable sheaths or screens to largely compensate the induction voltages such that the screen/sheath current and consequent losses are minimized in spite of the entire arrangement being grounded at both ends.
In the cross-bonding technique the cable arrangement is subdivided into three cable sections FCS, SCS, TCS and at two cross-bonding locations SCB1, SCB2 the screens 5 are cyclically connected such that the totally induced sheaths or screen voltage adds up to zero over the entire lengths, as shown in the bottom graph in FIG. 1.
Such techniques are described by Egon F. Peschke and Rainer von Ohlshausen in “Cable Systems for High and Extra-High Voltage”, Publicis-MCD-Verlag 1999, ISBN 3-89578-118-5, pages 58-62.